This invention relates to wireless communication systems, such as cellular packet networks, and more particularly to methods of and apparatus for improving efficiency in such systems during handoff.
In wireless communication systems for the transmission of data packets from a sending machine such as an Internet server, the switching or “handoff” of a mobile subscriber unit from one cellular base station to another is implemented as the subscriber unit moves between areas of different signal strength. Any discontinuities in the wireless data path as a result of a handoff can cause data packet loss, which results in missing or delayed acknowledgment signals between the end user machine and the server. This is true whether packets are destined for the end user machine or the server. This increases the likelihood that the applicable TCP protocols at either end of the network connection will invoke congestion avoidance/slow start modes at the server, leading to a drop in data throughput in the system.
In one arrangement for maximizing data throughput during periods of handoff, data packets destined for the subscriber unit are multicast to all of the base stations in the vicinity of the subscriber unit. Each of such base stations is identically tasked to store a succession of such data packets in an associated store-and-forward buffer or cache. When an actual handoff occurs from the base station then servicing the subscriber unit to a selected one of the other base stations with the identical cached packets, the selected base station forwards the stored packets in its own buffer to the subscriber unit. (In some cases, the use of additional “smart” facilities in the buffers to implement the so-called Snoop protocol can lead to further improvements in throughput).
While multicasting arrangements of this type can decrease the probability of lost packets to help maintain throughput in the system during handoff, they do not use the resources of the cellular system in an efficient way. All the base stations of the system that receive the multicast data packets from the server are tied up in the storage and processing of identical information for the same cellular customer, even though only one of such base stations will end up servicing such customer after handoff. The expensive facilities of all the other base stations in the group that are pressed into service in expectation of this particular handoff are unavailable for productive use elsewhere. In addition, by multicasting identical packets to so many different base stations, the load on the network infrastructure is unnecessarily increased.